Much progress has been made in the art of transmitting digital video signals for use by receiving terminals in a variety of applications such as video phones, video conference, and the like. High speed transmission services, such as T 1.5 carrier, ISN, 168 kilobit, 45 Megabit, ISDN and others are available to accommodate the high bandwidth required for video information transmission. In addition, data compression-decompression techniques are used to pack more video data into a given bandwidth which aids real time video and other applications.
Notwithstanding these and other advances, the high data content of full motion video prior art systems still require an undesirable technical trade-off between the nature of the transmission capacity and the informational content required per unit of time for satisfactory video images. Serious bandwidth constraints arise when transmission occurs over analog telephone lines or when color information is part of the video informational content.
Data compression techniques only partially relieve these problems by reducing somewhat the number of bits needed to convey video images. In some cases, these techniques introduce other quality problems such as less definition. haloing, shadowing, aliasing , imperfect diagonal lines, and noise generation that simulates information generation thus wasting bandwidth.
Today it is common practice to use 4-bit video as a method to reduce data rates in compressed video. In standard systems, the decommpression process attempts to restore the signal to its original 8-bit form. However, the number of gray scale levels in the ultimate display is determined by the smallest number of bits used anywhere in the signal path. The additional 4-bits are required by other techniques such as smoothing of edges, and/or removing other artifacts introduced by the compression/decompression process. Consequently, the standard receiver generates a 4-bit video signal representing the gray scale level of each pixel, i.e. a 16 level gray scale. Further, gray scale levels would require more bandwidth. Since the human eye perceives 20+ gray scale levels, standard systems trade off more discernable gray scale levels for the lower bit rate.